Seasonal variation in oxygenated organic molecules in urban Beijing and their contribution to secondary organic aerosol

Abstract

Abstract. Oxygenated organic molecules (OOMs) are crucial for atmospheric new particle formation and secondary organic aerosol (SOA) growth. Therefore, understanding their chemical composition, temporal behavior, and sources is of great importance. Previous studies on OOMs mainly focus on environments where biogenic sources are predominant, yet studies on sites with dominant anthropogenic emissions, such as megacities, have been lacking. Here, we conducted long-term measurements of OOMs, covering four seasons of the year 2019, in urban Beijing. The OOM concentration was found to be the highest in summer (1.6×108 cm−3), followed by autumn (7.9×107 cm−3), spring (5.7×107 cm−3) and winter (2.3×107 cm−3), suggesting that enhanced photo-oxidation together with the rise in temperature promote the formation of OOMs. Most OOMs contained 5 to 10 carbon atoms and 3 to 7 effective oxygen atoms (nOeff=nO-2×nN). The average nOeff increased with increasing atmospheric photo-oxidation capacity, which was the highest in summer and the lowest in winter and autumn. By performing a newly developed workflow, OOMs were classified into the following four types: aromatic OOMs, aliphatic OOMs, isoprene OOMs, and monoterpene OOMs. Among them, aromatic OOMs (29 %–41 %) and aliphatic OOMs (26 %–41 %) were the main contributors in all seasons, indicating that OOMs in Beijing were dominated by anthropogenic sources. The contribution of isoprene OOMs increased significantly in summer (33 %), which is much higher than those in the other three seasons (8 %–10 %). Concentrations of isoprene (0.2–5.3×107 cm−3) and monoterpene (1.1–8.4×106 cm−3) OOMs in Beijing were lower than those reported at other sites, and they possessed lower oxygen and higher nitrogen contents due to high NOx levels (9.5–38.3 ppbv – parts per billion by volume) in Beijing. With regard to the nitrogen content of the two anthropogenic OOMs, aromatic OOMs were mainly composed of CHO and CHON species, while aliphatic OOMs were dominated by CHON and CHON2 ones. Such prominent differences suggest varying formation pathways between these two OOMs. By combining the measurements and an aerosol dynamic model, we estimated that the SOA growth rate through OOM condensation could reach 0.64, 0.61, 0.41, and 0.30 µg m−3 h−1 in autumn, summer, spring, and winter, respectively. Despite the similar concentrations of aromatic and aliphatic OOMs, the former had lower volatilities and, therefore, showed higher contributions (46 %–62 %) to SOA than the latter (14 %–32 %). By contrast, monoterpene OOMs and isoprene OOMs, limited by low abundances or high volatilities, had low contributions of 8 %–12 % and 3 %–5 %, respectively. Overall, our results improve the understanding of the concentration, chemical composition, seasonal variation, and potential atmospheric impacts of OOMs, which can help formulate refined restriction policy specific to SOA control in urban areas.

Underestimated Contribution of Heavy Aromatics to Secondary Organic Aerosol Revealed by Comparative Assessments Using New and Traditional Methods

“…With these approaches, the OOM precursors are segregated into aromatic OOMs, aliphatic OOMs, isoprene OOMs, terpene OOMs, undistinguished OOMs, and nitrophenols. Note that the calculations of nO eff and the DBE value assume that the nitrogen atoms are only associated with the nitrate group (−ONO 2 ), which is reasonable based on the current knowledge on N-containing organic formation, as widely reported in both field and laboratory studies. ,,− …”

Exploring condensable organic vapors and their co-occurrence with PM2.5and O3in winter in Eastern China

“…In general, the concentration of atmospheric OOMs is extremely low (dozens ppqv to dozens pptv), 25 making it difficult to detect. This leads to a scarcity of global observations of OOMs.…”

Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NOx, and Degree of Oxygenation

“…The detected compounds were initially termed highly oxidized multifunctional organic compounds (HOMs) because of their high oxygenation levels observed in the first few pioneering studies . Since then, nitrate-CIMS has been widely used in both laboratory studies and field measurements to study HOM formation and their role in SOA production. ,− More recent nitrate-CIMS measurements − have shown compounds of lower oxygenation level than the originally defined HOMs, yet they share similar formation routes. Therefore, a broader term OOMs has been proposed to include most species detected by nitrate-CIMS and the term HOMs is reserved for compounds that fulfill the stricter definition.…”

“…Here, we study the molecular composition of OOMs by deploying a nitrate-CIMS in the urban center of Shanghai, China. The identified OOMs are first classified according to their possible precursors with a novel framework proposed in our recent work. , We then investigate the chemical characteristics and temporal variations of OOMs of different origins. Finally, factors that potentially affect the formation of OOMs, including precursor type, NO x level, and so forth, are discussed.…”

“…43,45,46 Field investigations in OOM formation from AVOCs are even scarcer. Recent field studies in urban Beijing 14,18,19,47 and Nanjing 17 indicated that high concentrations of NO x in urban environment might suppress the formation of OOMs, especially those with high oxidation states. 47 Our previous study in four megacities in China 48 also found that (1) AVOCs, mainly aromatic and aliphatic VOCs, dominated OOM formation, which significantly contribute to local SOA production through condensation and (2) NO x dominated the termination reaction with RO 2 radicals, and multistep bimolecular oxidation played a significant role in OOM formation in urban areas.…”